1. Field of the Invention
The present invention relates to a projection optical system, a projection unit, and a projection display device, and more particularly to an oblique projection type projection display device.
2. Description of the Related Art
A projection display device (hereinafter, called as “a projector”) for enlargedly projecting an image on a display device such as a liquid crystal panel onto a projection plane such as a screen has been commercialized and widely spread. A technique of shortening the distance between the screen and the projector main body has been developed in the field of projectors. Applying a wide-angle arrangement to a projection optical system and tilting a propagating direction of projection light with respect to an optical axis of the projection optical system is advantageous in shortening the distance between the screen and the projector main body.
For instance, there is proposed a projection optical system using a wide-angle lens having a large angle of view, and constructed in such a manner that a display device and a screen are shifted in directions opposite to each other with respect to an optical axis of the projection optical system. The above arrangement enables to shorten the projection distance.
There is also proposed a projection optical system using a refractive optics and a reflecting plane, and constructed in such a manner that an image on a display device is formed as an intermediate image at a position between the refractive optics and the reflecting plane, and the intermediate image is enlargedly projected onto a screen by the reflecting plane. The above arrangement is advantageous in further shortening the projection distance.
The following drawback, however, may occur in the projector using the wide-angle arrangement and the arrangement of obliquely projecting light. Specifically, in the case where a zoom function is added to the projector, the projecting position of the projected image on the screen may be greatly shifted depending on a change in a zoom state. As a result of the shifting, a part of the projected image may be deviated from the screen.
In the arrangement of enlargedly projecting the intermediate image by the reflecting plane, the forming position of the intermediate image may be greatly shifted depending on a change in a zoom state. In view of the shifting, a relatively large reflecting plane may be necessary to cover the entire area where the forming position is supposed to be shifted.